A coated steel strip obtained by coating a specific molten metal, e.g., molten zinc onto a cold-rolled steel strip or a hot-rolled steel strip, is highly corrosion-resistant and has aesthetic appearance.
For example, the steel strip coated with molten zinc has a greater zinc coating amount than an electrically plated steel strip. Thus the coated steel strip is highly corrosion-resistant and low in manufacturing costs. Also, lately the coated steel strip has been widely utilized for inner and outer bodies and an internal structure of automobiles due to superior coating cohesion.
FIG. 1 illustrates a conventional continuous galvanizing line, which is representative of a molten metal coating line.
As shown in FIG. 1, a coil steel strip S uncoiled from a pay off reel is thermally treated in a furnace through a welder and an entry looper. Then the coil steel strip S passes through a hot dipping bath 210 filled with molten zinc 212 through a snout 214 to be coated.
Next, the steel strip passes through a gas wiping apparatus or an air knife 220 disposed over a molten level of the hot dipping bath. In this case, the molten metal solution (zinc) of the steel strip S is adequately worn from the surfaces thereof by a high-pressure air or a non-active gas such as nitrogen, (hereinafter referred to as ‘gas’, blown onto the steel strip. This allows the steel strip to be adjusted in its coating thickness as shown in A of FIG. 1.
Afterwards, a gauge 220 measures whether the steel strip is coated with an appropriate coating amount. The measured value is fed back to adjust a gas ejection pressure of the gas wiping apparatus 220 and an interval between the steel strip S and the gas wiping apparatus 220, thereby continuously controlling a coating amount of the steel strip.
Here, reference signs 216 and 218 in FIG. 1, which are not described, indicate a sink roll for guiding the steel strip into the hot dipping bath and a stabilizing roll for suppressing vibration of the steel strip.
As described above, the gas wiping apparatus (air knife) 220 is the important equipment for determining a coating thickness of the steel strip to meet consumers' demand.
FIGS. 2(a) and (b) illustrate examples of a conventional gas wiping apparatus.
As shown in FIGS. 2a and 2b, the conventional gas wiping apparatus 200a and 200b includes a cylindrical or box-shaped chamber 202a and 202b, and upper and lower lips 230a, 240a, 230b and 240b joined to a front of the chamber and assembled in pairs in upper and lower parts thereof to form gaps G of gas slits or outlet 231 and 241 to eject the high pressure gas.
The high pressure gas ejected at a high speed onto the steel strip is fed at a high pressure through a feed pipe connected to the chamber, and finally ejected from the gas slit as a high speed jet flow (J of FIG. 2).
Meanwhile, in the gas wiping apparatus, a coating amount of the steel strip is adjusted by a wiping force for the molten metal deposited on surfaces of the steel strip which is submerged in a hot dipping bath (210 of FIG. 1) and then moves along with the molten metal.
As shown in FIG. 2a, the conventional gas wiping apparatus 200a may have a baffle wall 250 provided with passage holes 250a to eject the high pressure gas between upper and lower lips and the chamber in a width direction of the steels strip.
However, in the gas wiping apparatus 200a and 200b shown in FIGS. 2a and 2b, the cylindrical or square-shaped chamber supports the gas wiping apparatus against load and is fixedly connected to the upper and lower lips 230a, 240a, 230b and 240b, by welding or bolt/nut connections.
Therefore, to sustain strength of the chamber, the large-sized chamber should be sufficiently thick, thereby increasing an overall weight of the gas wiping apparatus. This weakens strength of the upper and lower lips and deforms the gas outlet, i.e., gap G between the upper and lower lips, causing the steel strip to be coated non-uniformly in a width direction of the steel strip.
Korean Patent Laid-open Publication No. 2004-0110831 filed by the same applicant of this application discloses another gas wiping apparatus having an improved structure for solving problems with the aforesaid conventional gas wiping apparatus. FIG. 3 illustrates such a gas wiping apparatus.
As shown in FIG. 3, this conventional gas wiping apparatus 300 includes a chamber 310, a baffle wall 322, a lip support unit 320 and upper and lower lips 340 and 350. The baffle wall 322 is fixedly associated with the chamber 310 and provided with passage holes 322a therein. The lip support unit 320 includes upper and lower lip support units 324 and 326 and has a predetermined thickness to sustain strength of the apparatus. The upper and lower lips 340 and 350 are fixedly connected to the lip support unit 320 to form a gas outlet, i.e., gap G.
Therefore, in the conventional gas wiping apparatus 300, the lip support unit 320 serves to support the apparatus against load. Also, the support unit 320 is fixed, at a front side, to the chamber with a smaller thickness and, at a rear side, to upper and lower lips in order to easily sustain strength of the apparatus.
However, in this gas wiping apparatus 300, the upper lip fixed to the lip support unit to adjust the gas outlet is very large-sized. Thus it is very difficult to install the upper lip in the lip support unit or adjust a gap between the upper and lower lips.
Especially, the gap is hardly formed in a gradient fashion in the conventional gas wiping apparatus 200 in order to prevent edge over coating (EOC) of the steel strip.
Moreover, the conventional gas wiping apparatus 300 is not designed in response to an ejection profile of a high-pressure gas fed to the chamber through a feed pipe.
Also, the chamber is not provided with a multistage uniform pressure space where the high pressure gas flows.
This has led the applicant to suggest an improved gas wiping apparatus.